Necroplasm is a will-controlled substance featured in the fictional Spawn

Spawn (comics)

Spawn is a fictional comic book superhero who appears in a monthly comic book of the same name published by Image Comics. Created by writer/artist Todd McFarlane, Spawn first appeared in Spawn #1...

 universe. It was originally indistinct/synonymous with psychoplasm, the substance of which Hell itself is composed. It is similar in properties, but special in that it can exist on Earth without needing to remain in one of Hell's spheres of influence as well as being allowed to traverse the mortal world without violating the non-aggression pact between Heaven and Hell. Necroplasm was a creation of the Grand-Mal Demon known as Leviathan and is exclusive to the 8th Sphere of Hell. He used it to create a demon in his own image, known as The Malebolgia

Malebolgia

Malebolgia is a fictional character in the Spawn universe, drawing its name from the term in Dante's Inferno used to describe Malebolge, the ditches in the eighth circle of Hell where humans who committed the sin Fraud are punished...

 (Mal), who turned on his master and usurped the 8th Sphere for his own. This gave Malebolgia full control over the necroplasmic reserves and thus the ability to create a unique army of warriors known as Hellspawn

Hellspawn

Hellspawn are fictional soldiers from Todd McFarlane's comic book series, Spawn. The main character in the series, Spawn, is himself a Hellspawn.-Description:...

 which he uses to tip the balance of power in Hell toward the 8th Sphere.

All Hellspawns in Todd McFarlane

Todd McFarlane

Todd McFarlane is a Canadian cartoonist, writer, toy designer and entrepreneur, best known for his work in comic books, such as the fantasy series Spawn....

's Spawn comic book series (and other media) are entirely composed of this substance. Necroplasm is one way that the otherworldly material of Hell may manifest physically in the real world of Earth. The main character of the series, Spawn (Al Simmons), wears a suit that is itself necroplasm-based and interacts with his fully necroplasmic body. It alters his physical appearance and allows him to use his body as a living weapon. The suit is a "living" symbiotic organism which provides him with various powers and great protection. He can also shoot beams of green necroplasmic energy from his body. Additionally, both his blood and eyes glow with the same green light, and also many of his magics, suggesting that raw necroplasm glows green.

Hellspawn are created with a limited amount of energy contained within their necroplasm, measured as 9,999 units to begin with. Once this energy is consumed, their time on Earth ends and the Hellspawn returns to Hell, his soul forfeit.

Etymology

The prefix necro- means "dead." It has its origin in the Greek nekro-, a combining form of nekrós, meaning "dead person," "corpse," or "dead."

The root -plasm describes the material forming cells or tissue. It comes from the Greek plasma, meaning "something molded or formed."

Thus, Necroplasm probably means "dead cells" or "formed of dead tissue." The word is a play on the term ectoplasm

Ectoplasm (paranormal)

Ectoplasm is a term coined by Charles Richet to denote a substance or spiritual energy "exteriorized" by physical mediums...

, which denotes a substance or spiritual energy that is "exteriorized" by physical mediums and said to be associated with the formation of ghosts.